Preclinical Modeling of T Cells in Immuno-Oncology

Classic syngeneic tumor models allow the study of endogenous T cell function 

Tumors that spontaneously arise or are experimentally induced in a mouse can sometimes be isolated and propagated in vitro. When these cell lines, such as B16F10 melanoma or CT26 colon carcinoma, are injected back into the compatible inbred mouse strain of origin (C57BL/6J or BALB/cJ in these examples), these syngeneic models allow the study of endogenous T cell function in fully immune competent mice.  

• University of Chicago researchers used the C57BL/6-derived B16F10 melanoma cell line to investigate the role of the microbiome in immunotherapy. Anti-PD-L1 therapy generated stronger tumor-specific T cell responses and intratumoral CD8+ T cell accumulation when the cells were implanted in the C57BL/6J mouse substrain from The Jackson Laboratory (JAX) compared to the C57BL/6Tac mouse substrain from Taconic Farms. Members of the genus Bifidobacterium, found in higher composition in the microbiome of the JAX mice, exerted these anti-tumor effects. 

• Mariathasan and colleagues selected the BALB/c-derived EMT6 mammary carcinoma cell line to study tumors that do not respond to anti-PD-L1 therapy. They found that co-administration of anti-PD-L1 with anti-TGFβ, which targeted tumor stromal cells, allowed for better T cell infiltration into the tumor and improved tumor regression.   

Basic T cell immuno-oncology and proof-of-concept studies are possible using syngeneics, but may lack translatability to human-specific molecules. However, translatability can be improved by generating new mouse models that genetically express human-specific variant molecules in place of their mouse counterparts. This can be done relatively quickly using CRISPR/cas9 technology to make the precise edits directly in the exact mouse genetic background of interest, ensuring the matching tumor cell line will still engraft with no immune rejection (JAX Custom Model Generation Services).  

Immunodeficient mice permit the study of human T cells and human cancers in vivo 

To generate human-specific therapies, permissive immunodeficient mouse strains, which lack the ability to mount an immune response, can be engrafted with tissues from human donors. Some sophisticated models include co-engraftment of both human immune cells and human cancer cell lines or Patient-Derived Xenografts (PDX) (JAX Onco-Hu Models). 

• Chimeric Antigen Receptors (CAR) have been used clinically to treat leukemias with success. However, the current versions can illicit unintended inflammatory immune reactions, because they contain fragments of mouse origin. To address this, Sommermeyer and colleagues employed the severely immunodeficient NSG mouse strain to test fully human CD19 chimeric antigen receptors (CARs) against a human leukemia. They successfully showed reductions in the immunogenicity. 

• Researchers at Merck chose both syngeneic models and NSG mice humanized with CD34+ hematopoietic cells to test the efficacy of anti-GITR on a human melanoma cell line. Treatment with anti-GITR antibody reduced the Treg population and activity within the spleen and tumor, allowing CD4+ and CD8+ T cells to increase production of antitumor effector cytokines. 

Immune system humanization continues to be improved with the addition of transgenic human cytokines, such as in NSG-SGM3 mice that express human IL3, CSF2, and KITLG. Additionally, when NSG mice are engineered to express human MHC molecules, such as NSG HLA-A2, and are paired with matched PDX and CD34+ cells, the humanized T cells can become appropriately MHC restricted. Depending on the application, NSG mice that lack endogenous MHC I and MHC II molecules (https://www.jax.org/strain/025216 ) can be used as hosts for human donor peripheral blood mononuclear cells (PBMC), reducing complications of Graft-vs-Host Disease (GvHD) and allowing rapid evaluation of human T-cell biology in vivo.